This invention relates to a method and apparatus for controlling the storage conditions of liquefied gases. It is of particular reference and benefit to the storage of liquefied natural gas (LNG) in ocean-going tankers.
Storing and transporting in liquid form such gases as natural gas and atmospheric gases offers considerable benefits in the large quantities that may be stored or transported in a given size of container. The low temperatures of such cryogenic liquids do however impose many severe requirements upon the container's design and operation. The container must be mechanically strong and capable of withstanding the low storage temperatures and the expansion and contraction stresses on heating and cooling between storage and ambient temperatures. It must be substantially if not entirely enclosed and provide a high level of insulation so as to minimise heat inleak and the resultant evaporation of the liquid.
The established use of a double-walled container with an interspace between the walls helps to achieve low heat inleak, and can be made more effective by the use of vacuum or other insulation in the interspace. Some heat inleak is nevertheless inevitable, leading to evaporation of the liquid. The heat inleak tends to cause a thermosyphon action within the container, liquid adjacent to the walls being warmed by the heat inleak and thereby becoming less dense and rising towards the surface. The upward movement adjacent to the walls correspondingly tends to impose a downward movement on the liquid at or near the centre of the container. The thermosyphon action makes it difficult to control the storage conditions. In particular when the warmer liquid rising near the walls reaches the surface it tends to boil, creating additional vapour and increasing the headspace pressure .
Additional means are generally required to reliquefy or otherwise deal with the vapours resulting from heat inleak. Venting of the evaporated material is generally undesirable and especially so in the case of natural gas because of its flammability and because its methane content and any other hydrocarbons it contains each function as greenhouse gases.
Various proposals have been made for retaining vapours within the container envelope. U.S. Pat. No. 3,918,265 describes an early process for reducing refrigeration losses from a plurality of storage compartments for low temperature liquid mixtures such as LNG, in which process liquid mixture is withdrawn from one of the compartments, is subcooled and then recycled into all of the storage compartments, with the proviso that a large portion of the subcooled mixture is recycled into the storage compartment from which the liquid mixture is withdrawn. The refrigeration value of the subcooled liquid is said to be sufficient to compensate for the loss of refrigeration values due to heat from the surroundings.
Introduction of a subcooled liquid as proposed by the said patent tends to add to the problems of maintaining controllable conditions within the container. For example the recycling of subcooled liquid may so inhibit the evaporation as to create a partial vacuum in the container's ullage space, with attendant risks of drawing in external materials. Drawing atmospheric oxygen into the container is particularly to be avoided because of the danger that it could lead to a combustible or explosive mixture within the container. A related problem is that the partial vacuum may impose undue stress on the container structure.
The recycling of subcooled liquid may also encourage stratification within the stored liquid. The subcooled material being more dense than the stored bulk tends to sink to form a dense lower layer and to encourage the formation of successively lighter layers towards the liquid surface. The light top layer is then particularly prone to evaporation. Moreover the evaporation of the lighter fractions from the top layer increases its density relative to the lower layers and can lead to a sudden rollover and mixing of the layers which may result in a violent boiling action.
Solutions for controlling vapours resulting from heat inleak have therefore generally been sought in reliquefying the vapours and returning them to the stored bulk. These introduce other problems with LNG, which is primarily a mixture of methane and nitrogen, in that the composition of the vapour (otherwise known as “boil off”) is different from that of the liquid and generally has a much higher proportion of nitrogen. The higher the nitrogen content of the boil off, the more difficult is its reliquefaction. The nitrogen content of the boil off varies according to the composition of the transported LNG. The higher the mole fraction of nitrogen in the boil off the lower is the pressure and temperature to which the refrigerant is expanded in order to achieve its total reliquefaction.
Reducing the pressure to which the refrigerant is expanded leads to a larger and more costly refrigerator with higher power consumption. Indeed, since the nitrogen content of the boil off can fluctuate quite appreciably dependent on the transported LNG composition, in order to be sure of totally liquefying the boil off, the refrigerator has to be designed in order to meet the least favourable circumstances, as may exist in the LNG spot market. The conventional solution to this problem is to vent a part of the boil off and therefore restrict the size of the refrigerator. As mentioned above, this solution is environmentally unacceptable. It must be also noted here, that the refrigerator for reliquefying vapour must handle the vapour compression heat in addition to the heat inleak only. This increases the refrigerator size by 20 to 30%.
Moreover because the reliquefied natural vapours have a higher nitrogen content they have a higher density than the stored bulk. This further increases the likelihood of stratification as the heavy recycled material sinks towards the bottom of the container.